Heart disease is one of the leading causes of death for both men and women throughout the world. According to the American Heart Association, an estimated one in three adults (80.7 million) have one or more types of cardiovascular disease of whom 38.2 million are estimated to be age 60 or older. Worldwide, coronary heart disease kills more than 7 million people each year. The United States Center for Disease Control (CDC) estimates that 47% of all cardiac deaths in the United States occur before emergency services are received or before the patient can be transported to a hospital.
In addition, in patients having coronary arterial disease, only approximately 40% experience symptoms such as angina, chest pain, shortness of breath, and light headedness. However, when an ECG (electrocardiogram) is taken of the patient's heart, many do not show a positive change in the current resting ECG, thus making it extremely difficult to identify if there is a level of myocardial ischemia (restriction of blood flow to the heart) present. According to the American Heart Association, at present, approximately 67% of all coronary arterial diseases are correlated to some degree of myocardial ischemia. Further, over 60% of patients with coronary arterial disease may not feel the symptoms of myocardial ischemia (i.e. they are asymptomatic) such that they do not visit their physician or clinic. 50 percent of men and 64 percent of women who died suddenly of coronary heart disease had no previous symptoms of this disease. This year an estimated 770,000 Americans will have a new coronary attack and about 430,000 will have a recurrent attack. It is estimated that an additional 190,000 silent first heart attacks occur each year. (NHLBI: Based on unpublished data from the ARIC and CHS studies.)
Cardiovascular testing represents an integral component of care for at-risk patients, both at the diagnostic stage and during patient care in a medical facility. According to data obtained from the Centers for Disease Control and Prevention's Annual National Hospital Discharge Survey, approximately 4.3 million patients were discharged from a hospital with a diagnosis of heart disease in 2006. In addition to the approximately 4.3 million patients discharged from a hospital with a diagnosis of heart disease in 2006, another approximately 15.3 million individuals received an ECG test in an ambulatory emergency clinic or hospital emergency department. Physicians also had approximately 23.7 million individuals tested with an ECG monitor and another approximately 2.1 million individuals received an ECG test in an outpatient care setting such as a cardiac rehabilitation center, according to the Centers for Disease Control and Prevention.
The number of heart related testing is expected to increase in tandem with the aging of the population, both at the diagnostic stage and during acute care. Trends toward preventive care and maintenance will also mean that more individuals diagnosed as high risk for a cardiac condition will receive more routine tests to monitor the progress of the condition. These trends, in tandem with the push to shorten hospital stays, have created an impetus to identify risks earlier in pre-symptomatic patients at the physician or clinic level and to treat recovering cardiac patients in cardiac rehabilitation centers.
Currently there are many tests that are used in connection with cardiovascular conditions. One common test that is performed is an angiogram which requires that a catheter be fed through an artery or vein into the area to be studied through which a dye is injected to make the blood flowing inside the blood vessels visible on an x-ray. An angiogram can show if coronary artery disease is present and how severe it is. However, angiograms are invasive, expensive and dangerous to perform.
Another test that is commonly performed is the non-invasive ECG. Current ECG technology detects approximately 1000 electrical impulses generated by the heart during each heart beat via skin electrodes and processes the impulses over time to create a one dimensional waveform that represents the heart. Specifically, the ECG breaks down each heartbeat into a series of three distinct electrical waves: the P wave, the QRS complex and the T wave. The P wave represents the activity in the heart's upper chambers while the QRST complex and T wave represent the activity in the lower chambers.
During an ECG, electrodes are placed on different sides of the heart to measure the activity of different parts of the heart muscle. The ECG displays the voltage between different pairs of these electrodes, and the muscle activity that they measure, from different directions, also understood as vectors. The ECG waveform indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle. It can measure and detect abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by levels of dissolved salts (electrolytes), such as potassium, that are too high or low. While an ECG can assist in identifying damaged heart muscle, it can only identify damage to muscle in certain areas of the heart. Further, it cannot reliably measure the pumping ability of the heart.
Many people with coronary artery disease, heart valve disease or heart muscle disease will eventually show abnormal ECG readings. However, abnormalities that occur may not show up. Because it is very common to see this false-negative result (e.g., the EKG does not find the damage or abnormality that is really present), a normal ECG is not enough to rule out suspected heart disease.